1. Field of the Invention
The present invention relates to a process for producing storage-stable surfaces of polished silicon wafers by oxidative treatment and subsequent exposure to organosilicon compounds.
2. The Prior Art
The separation in space and time between the production of semiconductor wafers as a starting material and their further processing to produce electronic components as the true final product often results in transportation and storage times of different lengths for the wafers. At the same time, the component producers demand that, regardless of this period of time and the influences active in the course thereof, the semiconductor wafers used always have an identical and reproducible surface quality and thereby guarantee a reproducible behavior in the sensitive production processes.
On the other hand, it is known that the perfect wafer surface produced in the polishing operation is exposed, even immediately after the latter has been completed, to a wealth of influences, for example, water due to washing solutions and, in particular, the ambient atmosphere, which may affect the surface quality. In this connection, the atmospheric effects during transportation and storage, for example, due to the varying period of time and the climatically and/or seasonally induced differences, which are often great, between the different producer countries (temperature, atmospheric humidity and contamination of the atmosphere with, for example, hydrocarbons) deserve special attention. This is because they may result in varying behavior of the wafers in the initial tests and/or some process steps during the component production.
Even the process specified by W. Kern and D. Puotinen in RCA Review, Jun. 1970, pages 187-206 (so-called "RCA cleaning"), in which the polished wafer surface is coated with a thin layer of partially hydrated silicon dioxide by oxidative treatment, for example, by immersion in a hydrogen peroxide/ammonia solution, cannot, as a rule, permanently prevent the contamination of the surface with the virtually ubiquitous hydrocarbons. In addition, as a result of aging effects, the structure of the protective layer also alters, especially during prolonged storage times. During epitaxial processes or thermal oxidation steps, for example, this may result in a misty cloudiness ("haze") on the polished surface, which manifests itself as a diffuse surface reflections when, for example, the wafer surface is examined in focused light.
According to German Offenlegungsschrift 3,540,469, of the corresponding U.S. Pat. No. 4,724,171, this effect can be prevented by treating the oxidized silicon surface with hexamethyldisilazane, the protective action being explained by the OH groups of the wafer surface reacting by trimethylsilylation and, consequently, being withdrawn from the aging process which presumably takes place with the condensation of water between the various OH groups. Silicon wafers treated in this way exhibit an excellent storage stability, but have a hydrophobic wafer surface which is held in little esteem by component producers and which, in addition, is still comparatively carbon-rich (theoretically, three methyl groups per bound silicon). A water drop applied to such a hydrophobic wafer surface, for example, contracts and often assumes even an almost spherical shape, whereas, on a hydrophilic wafer surface, it spreads and sometimes even drains off. The wetting angle between the wafer surface and the drop surface can be used to test and at least approximately measure the hydrophilic or hydrophobic surface properties and their change during storage.